Aging is associated with an increased incidence of hypertension and other cardiovascular diseases. Premenopausal women have a lower rate of cardiovascular disease than age-matched men or postmenopausal women. Since estrogen replacement therapy in postmenopausal women provides significant protection against the development of cardiovascular disease, it has been hypothesized that estrogen has beneficial effects on the cardiovascular system. The focus of this proposal is the study of the effects of age (progressing from young adulthood to middle age) and long-term in vivo estrogen on the vasodilator junctions of the vascular endothelium. Experiments are designed to test three hypotheses: 1) responsiveness to vasoconstrictors will increase with age and responsiveness to endothelium- dependent vasodilators will decrease with age; 2) the relative importance of various endothelial mediators of vasodilation (nitric oxide, endothelium-derived hyperpolarizing factors, and cyclooxygenase products) will change with age; and 3) estrogen will enhance agonist-induced endothelial nitric oxide production in arteries from young rats and enhance the production of endothelium-derived hyperpolarizing factors in middle age rats. These hypotheses will be tested by studying endothelium- dependent vasodilations to acetylcholine and a selective histamine H/1 agonist in small mesenteric arteries from young (13 weeks) and middle age (9-10 months) female rats. Three groups of rats of each age will be studied: ovary-intact, ovariectomized, and ovariectomized plus estrogen replacement. Pharmacological inhibitors of known endothelium-dependent vasodilator pathways (nitric oxide, cyclooxygenase products of arachidonic acid metabolism, and endothelium-derived hyperpolarizing factors) will be employed to determined the contribution of each pathway to endothelium- dependent vasodilation in young and middle age rats, and to further determine the effects of estrogen on these pathways. Western blot analysis for endothelial nitric oxide synthase (eNOS) will be used to determine whether tissue levels of eNOS protein change with age or estrogen therapy. The results of these studies will provide important new information regarding the in vivo effects of estrogen on a background of aging arterial and endothelial function. If the precise mechanisms by which estrogen affords its beneficial effects on the cardiovascular system can be determined, perhaps new therapeutic agents can be designed that will specifically target the cardiovascular system that effects on other estrogen-responsive tissues.